The present invention is generally in the field of printed circuit board packaging. More specifically, the invention is in the field of over-molded integrated circuit xe2x80x9cICxe2x80x9d) packages on printed circuit boards.
Metal shields are used on a printed circuit board to protect sensitive devices on the printed circuit board from electric fields emanating from other devices present on the printed circuit board or from the surrounding environment. The metal shield is generally connected to ground on the printed circuit board. Thus, the metal shield blocks electric fields by grounding any electric fields that contact the metal shield, thus preventing them from entering the area on the printed circuit board enclosed by the metal shield, also referred to as the xe2x80x9cenclosed areaxe2x80x9d. These metal shields are fabricated from metals such as stainless steel, copper, or copper alloys.
In a known surface mount over-molded printed circuit board package manufacturing process, passive devices such as capacitors are attached to the printed circuit board first. After the passive devices have been attached, a semiconductor die may be attached within the same area on the printed circuit board where the passive devices are located. Wire bonding is then performed between die bonding pads on the semiconductor die and printed circuit board bonding pads.
Generally, after wire bonding and prior to mounting of the metal shield over the semiconductor die and passive devices, a glob of epoxy coating is used to cover the semiconductor die and the bond wires to protect the die from moisture and other contamination. The metal shield is then mounted on the printed circuit board and encloses the xe2x80x9cglob toppedxe2x80x9d semiconductor die and the passive devices. The metal shield is typically attached to the printed circuit board by soldering it (i.e. the metal shield) to an exposed ground interconnect on the board. The introduction of the xe2x80x9cglob topxe2x80x9d epoxy involves dispensing precision thin epoxy over the die and bond wires and as such is a slow and expensive process.
There are other problems besides the long time and expense associated with the known method described above for shielding the semiconductor die and discrete passive components on the printed circuit board. One problem is that the metal shield may not be securely attached to the board or may loosen over time due to breakage or abuse. Also, if pressure is applied to the metal shield it may be deformed and it may contact the passive devices that it is shielding, resulting in electrical shorts or opens.
Another problem with the known method described above is that generally only the semiconductor die is encapsulated in the epoxy compound. The other devices under the metal shield are still subject to moisture and contamination that may enter the enclosed area through openings in the metal shield. In addition, it is sometimes difficult to control the application of the epoxy compound to the semiconductor die. As a result, there may be bond wires that are not encapsulated and therefore remain exposed, resulting in a higher likelihood of damage to the bond wires.
Thus, it is seen that there is a need in the art for a method and structure to overcome the problems associated with the presently known methods for shielding a semiconductor die and neighboring discret passive components used in a printed circuit board.
The present invention is directed to molded shield structures and method for their fabrication. The invention""s method and structure overcomes the problems associated with the presently known methods for shielding an enclosed area on a printed circuit board.
According to one embodiment of the invention, a die is attached to a printed circuit board, for example, by using a die attach epoxy. Thereafter, at least one post is attached to the printed circuit board adjacent to the die. The at least one post can be, for example, made of copper or solder-coated ceramic.
A shield is then mounted on the at least one post. The shield encloses the area on the printed circuit board which includes the die and may include additional elements, such as passive devices. The shield can be, for example, made of stainless steel, copper, or a copper alloy. In one embodiment, the shield is a wire mesh which can be made of copper or aluminum. After mounting the shield, a molding compound is used to encapsulate the shield, the at least one post, the die, and any additional elements, such as passive devices, that are enclosed by the shield.